Conventional asphalt cement (AC) produced by a petroleum refinery must be modified to enhance its qualities as a binder. Asphalt cement derived from petroleum refining is a mixture of hydrocarbons and heterocyclic compounds, including asphaltenes and maltenes formed from polynuclear hydrocarbons of relatively high molecular weight. Asphalt varies depending upon its source, i.e., the crude oil and the refinery. An asphalt binder is defined as the substance used to hold together the aggregate particles, e.g., crushed rock, stone, fillers, etc., and thereby form the asphalt concrete.
Various modifiers have been disclosed and adopted for commercial use in an effort to improve the performance and extend the life of asphalt paving compositions. One class of material that has been added to the hot asphalt are polymer modifiers. One commonly used polymer that is added to the asphalt at high temperature is styrene-butadiene-styrene (SBS) block co-polymer in a range that is typically less than 10% by weight of the asphalt binder mixture. Although certain physical properties and performance characteristics of the applied asphalt concrete are improved, the use of SBS has several drawbacks, including its insolubility in the asphalt and that it separates and rises to the top of the asphalt during storage unless the mixture is subjected to the appropriate type of mixing on a continuous basis. When used with GTR, the asphalt-SBS mixture is subject to an increase in viscosity during storage. A further drawback to the use of SBS as a modifier is that relatively higher levels of the polymer are required to achieve the same level of improvement in the federally-established Strategic Highway Research Program (SHRP) asphalt binder test ("PG") rating.
Polypropylene has also been used commercially as a modifier, but like SBS this material does not form a true solution with the asphalt and if the mechanical stirring and/or recirculation of the mixture is discontinued or drops below a certain level, the polymer additive will separate to form a separate layer on the surface of the hot asphalt in the processing equipment.
Another polymeric material that has been suggested for use as a modifier of asphalt is styrene-butadiene rubber (SBR). However, the modified asphalt containing SBR produces a composition having relatively high tack which results in an undesirable build-up of the material on paving machines' flights, hoppers, shovels and lutes. This effect is also manifested as a pick-up on the surface of the roller equipment when the hot concrete is being finished on the road surface which can only be mitigated by allowing the asphalt to cool to less than about 150.degree. F. before being rolled. In addition, the finished paving concrete has a greater tendency to develop "spider webs", or fine surface cracks, under certain climatic conditions.
A variety of other polymers, including rubbery terpolymers, have been disclosed as having utility as asphalt modifiers. For example, U.S. Pat. No. 5,733,955 discloses the addition of a rubbery polymer as a modifier that is comprised of a conjugated diolefin monomer, a vinyl aromatic monomer, and isobutoxymethyl acrylamide that form the backbone of the polymer. An asphalt modifier is disclosed in U.S. Pat. No. 5,773,496, that is a mixture of sulphur and a polymer comprised of a linear co-polymer that is a styrene-conjugated diene block co-polymer and a similar linear co-polymer of specified molecular weight. It does not appear that these or other polymeric modifiers have been adopted for use by commercial paving contractors.
A material that has been adopted for use as an asphalt cement modifier is crumb rubber. The principal source of crumb rubber in the United States and elsewhere throughout the world is ground tire rubber (GTR) from discarded tires. The U.S. Federal Highway Administration (FHWA) has approved crumb rubber produced from scrap tires for use in highway pavements as a modifier for asphalt cement. The use of crumb rubber as a modifier for asphalt cement in place of other "virgin" polymeric modifiers has been hailed by some as a solution to the ever-growing problem of disposing of discarded tires in an ecologically acceptable and economical way.
Crumb rubber, or GTR, of varying particle sizes has been incorporated into the hot asphalt cement, although relatively small particle sized materials of 40 mesh or smaller are preferred. Two basic types of crumb rubber are available and are identified by the processes for their respective manufacture. Cryogenic GTR is produced by shredding the tire into relatively large pieces and then subjecting the rubber to grinding under cryogenic conditions. Ambient or warm-ground GTR is produced under non-cryogenic conditions. The cryogenic GTR exhibits a certain regularity of shape when examined by scanning electron microscope (SEM) techniques and reveals particles having generally planar surfaces of relatively low surface area, as might be expected from the fracturing of the cryogenically frozen crystalline tire rubber. In contrast, ambient GTR particles are irregular in appearance with extended tendrils resulting from the pulling apart and shredding of the rubber structure (as compared to cryogenic fracturing) and possess a much higher surface area as compared to the particles produced by the cryogenic process.
The use of GTR as a modifier for asphalt cement that is used as a binder in paving compositions provides several advantages, including a more resilient road surface that flexes under the force of passing vehicular traffic to break up thin sheets of ice during formation. The road surface is also darker and therefore absorbs more of the sun's radiant energy to melt accumulating ice and to more quickly dry a wet surface. The addition of GTR also provides improved skid resistance. However, there are limited opportunities for a chemical bond to form between the asphaltenes and maltenes and the rubber particles and it is difficult to uniformly disperse the GTR in the asphalt cement. As a result of these chemical and physical limitations, finished pavement containing GTR is still subject to rutting under continuous traffic and/or heavy loads, and also to shoveling in areas of braking and rapid acceleration.
It is therefore an object of the present invention to provide an improved rubber modified asphalt paving binder that enhances the performance characteristics of asphalt paving compositions containing crumb rubber or GTR.
Another object of the invention is to provide an improved process for the manufacture of rubber modified asphalt paving binders.
It is also an important object of the invention to provide an improved process and composition for roadway pavements that will foster the widespread adoption and use of GTR from discarded tires and thereby advance the recycling of discarded tires to resolve a major environmental and ecological problem in the United States and elsewhere throughout the world.
A further object of the invention is to provide an improved rubber modified asphalt paving composition that can be produced in existing asphalt mixing equipment and paving machines.
Another object of the invention is to provide an asphalt binder having enhanced properties due to cross-linking that will produce a superior asphalt concrete when applied to poorly prepared subsurfaces.
It is yet another object of the invention to provide an improved rubber modified asphalt paving binder that employs commercially available polymeric modifiers and that is more economical to produce and apply than currently used polymeric modifiers and additives.